<!DOCTYPE html>
<html lang="zh_CN">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

</body>
<script type="text/javascript">
    var username = 'an';


    // var status = username.length >= 3 && username.length <= 18;

    /* username.length < 3 || username.length > 18 这个如果是成立，代表不满足我们的业务条件（必须在3-18个字符之间）。所以取反面代表满足业务*/
    var status = !(username.length < 3 || username.length > 18);

    // 2 >= 3 && 2 <= 18;
    // false && true ---> false

    console.log(status);



</script>

</html>
